Current level switching apparatus for operating electric discharge lamps



Aug. 9, 1966 w. F. POWELL, JR 3,265,930

CURRENT LEVEL SWITCHING APPARATUS FOR OPERATING ELECTRIC DISCHARGE LAMPSFlled May 5, 1962 9 Sheets-Sheet l I l I l 1 I a I k l 1 25- 26- -27 a I1 l I I INVENTOR.

WALTER F. POWE LLJR savvy M ATTORNEY Aug. 9, 1966 w F. POWELL, JR3,265,930

CURRENT LEVEL SWITCHING APPARATUS FORVOPERATING ELECTRIC DISCHARGE LAMPSFiled May 1962 9 Sheets-Sheet 2 2 E e [I D Q G 5 b d f G 'r TIME :ElEF-4 30 31 F l 332 am /34 I I 2 E |o e E 1 AlllL c I VY" Q3 5 fi l 31 R -37ss- 39- OT-'-" L. I L

INVENTOR.

WALTER E POWELLJR. BY 704;.7 W

ATTOR NEY Alig- 1966 w F. POWELL, JR 3,265,930

CURRENT LEVEL SWITCHING APPARATUS FOR OPERATING ELECTRIC DISCHARGE LAMPSFlled May 5, 1962 9 SheetsSheet 5 LAMP CURRENT TIME INVENTOR. WALTER FPOWELLJ R.

ZQM XM ATTORNEY Aug. 9, 1966 w. F. POWELL, JR 3,265,930

CURRENT LEVEL SWITCHING APPARATUS FOR OPERATING ELECTRIC DISCHARGE LAMPSFiled May 3' 1 2 9 Sheets-Sheet 4 LAMP CURRENT wAvEFoRM I BEFOREVARIATIONS 5 INDICATED IN FIGS. 8, 9, 2E v AND IO wERE MADE j TIMEElf-E1... E

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INVENTOR.

WALTER E POWELLJR.

MA- W ATTORNEY Aug. 9, 1966 w. F. POWELL, JR 3,265,930

CURRENT LEVEL SWITCHING APPARATUS FOR OPERATING F l d M ELECTRICDISCHARGE LAMPS i e a 3, 19

y 62 9 Sheets-Sheet 5 vvvvv DRIVER INVENTCR. WALTER F. POWELLJR.

ATTORNEY w. F. POWELL, JR CURRENT LEVEL SWITCH Aug. 9, 1966 3,265,930

ING APPARATUS FOR OPERATING ELECTRIC DISCHARGE LAMPS 9 Sheets-Sheet 6Filed May 5, 1962 .rzmmmso TIME INVENTOR. WALTER F POWELLJR.

ATTORNEY Aug. 9, 1966 w. F. POWELL, JR 3,265,930

CURRENT LEVEL SWITCHING APPARATUS FOR OPERATING ELECTRIC DISCHARGE LAMPSFiled May 5, 1962 9 Sheets-Sheet 7 E 1E; l E

'l 8| BIDIRECTIONAL I as L} SWITCH 87 M I 90- 9192 I FELDB/{CK LBIDIRECTIONAL /ae WTCH QU- CONTROL( 8 I 93 ,94 88 l a9 a2 95 C v v v v v8 4 INVENTOR.

WALTER F POWELLJR.

ATTORNEY Aug. 9, 1966 w. F. POWELL, JR 3,265,930

CURRENT LEVEL SWITCHING APPARATUS FOR OPERATING ELECTRIC DISCHARGE LAMPS9 Sheets-Sheet 9 Filed May 5, 1962 INVENTOR. WALTER F POWELLJR.

ATTORNEY QVE b i h b 0 u n L 5 U 13.3w .8. .0 1 i Ezoiowmm m9 wo h o 33%0 United States Patent 3,265,930 CURRENT LEVEL SWITCHDJG APPARATUS FOROPERATING ELECTRIC DISCHARGE LAMES Walter F. Powell, Jr., Danville, IlL,assignor to General Electric Company, a corporation of New York FiledMay 3, 1962, Ser. No. 192,231 24 Claims. (Cl. 315-209) This inventionrelates generally to apparatus for operating electric discharge devicessuch as fluorescent lamps, and more particularly, it relates to animproved ballasting and operating arrangement for such apparatus.

An electric discharge lamp usually includes an enclosure containing agas or vapor and a pair of electrodes. If the electric discharge lamp isof the fluorescent type, the enclosure is coated with a fluorescentpowder. An electric discharge must be initiated within the enclosure toprovide radiation at a wave length at which the fluorescent powdercoating is sensitive to excite the coating to a state of luminescence.Thus, when the electric discharge within the enclosure of the lamp isinitiated, light is generated, and the lamp is usually referred to asbeing started or ignited.

Essentially, the functions of the apparatus used to operate the electricdischarge lamp or the so-called ballast are to start, maintain andcontrol the electric discharge within the lamp enclosure. It will beunderstood that the electric discharge is initiated when a stream ofpositive ions within the enclosure moves generally from the anode to thecathode and a counter stream of electrons moves from the cathode to theanode. Where the electric discharge lamp is energized with alternatingcurrent, the electrodes alternate as anode and cathode as the polarityreverses in each half cycle.

The positive ions in the lamp enclosure are produced by ionizingcollisions between electrons and the neutral particles of gas or vaporparticles. A minimum amount of energy must be continuously furnished atthe electrodes to cause sufficient ionizing collisions to occur in orderto maintain the electrical conductivity of the discharge. These ionizingcollisions in the discharge are necessary since the electron flow aloneis not suflicient to produce the energy levels required for theproduction of light. Since the ionizing collisions cause free electronsto be liberated from the neutral particles, these free electrons areavailable to cause additional ionizing collisions. Unless the ionizingcollisions in the electric discharge are controlled by the externaloperating apparatus, the current or electron flow at the electrodes willbecome excessive and destroy the lamp. This tendency of the ionizingcollisions in the electric discharge to produce an excess of electronscauses the lamp to have a terminal to terminal dynamic negativeresistance characteristic. To control the ionizing collisions in theelectric discharge, the operating apparatus must limit the energysupplied to the lamp.

In conventional circuits used to operate electric discharge lamps withdirect current, the energy supplied to the lamp is limited by placing aresistor in series circuit with the lamp. Where the electric dischargelamp is operated with alternating cur-rent, usually an inductor or acombination of inductor and capacitor connected in series circuit withthe lamp serves as a ballasting element. The lamp, the ballastingelement and driving voltage source form a closed series loop. When theionization collisions in the electric discharge increase, the current inthe external circuit and the voltage drop across the ballasting elementincrease. The voltage drop across the lamp decreases since the sum ofthe voltage drops in the circuit must be equal to the driving voltagewhich is fixed. Thus, the energy level at the lamp is reduced and theionizing collisions in the electric discharge are diminished.

When the ionizing collisions in the electric discharge decrease, thiscondition is accompanied by a decrease in the electron or current flowout of the lamp thereby causing a proportional decrease in the voltagedrop across the ballasting element. This decrease in voltage causes acorresponding decrease in the potential or energy level at the lamp.Consequently, the ionizing collisions in the electric discharge areincreased. In this manner, control of the ionizing collisions in theelectric discharge is maintained.

An inherent disadvantage of conventional A.C. ballasting systems is thatthe peak energy stored in the reactive elements in the system must bemaintained at relatively high levels to perform the ballasting functionand the relative rate of storage and release of this energy is low. Thisresults in the use of heavy, bulky reactive devices, which arerelatively expensive. A disadvantage of conventional D.C. ballastingsystems is that a relatively large amount of power, generally aboutfifty percent of the power supplied, is dissipated in the ballastingresistor.

In a conventional 60 cycle ballast an open circuit voltage must beprovided that is not only suflicient to provide the necessary startingpotential for the electric discharge lamp but must also provide suitableregulation and stability characteristics for the ballast. If thedifference between the open circuit voltage and the normal lampoperating voltage in a conventional ballast is small, slight changes inthe supply voltage will produce appreciable variations in the lightoutput of the lamp. Generally, a ballast for operating a single 40 wattrapid start fluorescent lamp will provide an open circuit voltage whichis about twice as large as the operating voltage of the lamp. It will beappreciated that if the difference between open circuit voltage and thelamp operating voltage in a ballast is reduced and if the peak energyrequirement of the storage element is reduced, it is possible to employa smaller and less expensive storage element in the ballast. It isdesirable, therefore, to provide an apparatus for operating electricdischarge lamps that does not require a large difference between theopen circuit voltage and lamp operating voltage and that does notrequire an appreciable amount of energy to be stored in the storageelement of the system.

One of the continuing problems associated with illumination by electricdischarge lamps is the reduction in the light output that occurs as theelectric discharge lamps age. For example, a 96 PG 17 power groovefluorescent lamp will provide a 100 percent light output in the initialperiod of its life. After 2000 hours of operation the light outputreduces to approximately percent of its initial light output and after10,000 hours of operation the light output decreases to approximately 50percent. It is, therefore, desirable that an apparatus for operatingelectric discharge lamps be readily adaptable to regulation by feedbackmeans so that the light output of the lamps can be maintained by thefeedback means at a substantially constant value during their normallife.

Where the electric discharge lamp is operated with direct current, aconventional resistive ballasting element introduces an appreciableelectrical loss into the system that reduces the over-all electricalei'liciency of the system. In both direct current and alternatingcurrent ballasting 35 systems, there is a need for improvement inelectrical circuit efficiency.

Accordingly, it is a general object of the present invention to providean improved apparatus for operating an electrical discharge device.

Another object of the present invention is to provide an improvedapparatus for operating electric discharge lamps, such as a fluorescentlamp, wherein the peak energy stored in the apparatus is relativelysmall as compared with the peak energy stored in conventional 60 cycleballasts.

It is still another object of the present invention to provide animproved apparatus for operating and ballasting electric discharge lampsthat is readily adaptable to regulation by feedback means.

A more specific object of the invention is to provide an improvedapparatus for operating electric discharge lamps that does not require asubstantial diiference between the open circuit voltage and theoperating voltage of the electric discharge lamp.

It is a further object of the present invention to provide an improvedapparatus for operating electric discharge lamps wherein the electricalefliciency can be readily designed into the apparatus.

Still a further object of the present invention is to provide animproved apparatus capable of operating electric discharge lamps at asubstantially constant light output during the life thereof.

It is another object of the present invention to provide an apparatusfor operating one or more fluorescent lamps that does not require theuse of relatively heavy and bulky reactive devices such as high leakagereactance transformers.

In the broader aspects of the invention, an improved apparatus foroperating one or more electric discharge devices, such as fluorescentlamps, is provided wherein the electric discharge of the device iscontrolled and maintained by repetitively activating a switching meansto cause excursions of electrical current to be alternately suppliedfrom the power source and a storage element in the operating circuit.Stabilization of the electric discharge of the device is achieved byrepetitively switching the current between controllable levels.Preferably, the current levels are regulated in response to the changein various parameters sensed or detected by a feedback means. Forexample, lamp current, light output, lamp voltage, lamp power, linevoltage, lamp hot spot temperature, ambient temperature, outdoorillumination or any combination of these parameters may be sensed by thefeedback means for the purpose of controlling the switching action.

According to another aspect of the invention, an improved apparatus isprovided for operating one or more electric discharge lamps from a powersource wherein the supply voltage is stepped down to provide the voltagerequired to maintain the electric discharge of the lamp. The electricdischarge of the lamps is stabilized by repetitively switching theapparatus to provide controlled excursions of current alternately fromthe power source and from the storage element in the apparatus.

In another form of my invention, I provide an improved apparatusembodying the current level switching arrangement of the inventionwherein the supply voltage applied at the input of the apparatus isstepped up in magnitude to provide a relatively increased voltage acrossthe lamp. In this embodiment of my invention, a first storage element isused to step-up the supply voltage when the power source is switched into energize the lamp, and a second storage element is used to provide anexcursion of current to the lamp when the power source is switched outof the lamp circuit and supplies energy to the first storage element.

I have also provided an improved apparatus incorporating the currentlevel switching arrangement of the invention and supplying abidirectional current to operate i an electric discharge lamp. In thisembodiment of the invention, the current level switching action isprovided in both the positive and negative half cycles to maintain andstabilize the electric discharge. Preferably, the current levels arecontrolled in response to variations in the current, light output, orboth.

The subject matter which I regard as my invention is set forth in theappended claims. The invention itself, however, together with furtherobjects and advantages thereof may be better understood by referring tothe following description taken in conjunction with the accompanyingdrawings in which:

FIGURE 1 is a simplified schematic circuit diagram illustrating oneembodiment of my invention wherein a free-running driver is used torepetitively switch the power source in and out of the lamp operatingcircuit in a voltage step-down arrangement;

FIGURE 2 is a schematic circuit diagram of the driver shown in blockform in the circuit diagram of FIGURE 1;

FIGURE 3 depicts an oscillogram of the waveform of the lamp currentproduced by the apparatus shown in FIGURE 1;

FIGURE 4 illustrates a preferred form of my invention wherein theswitching means of the improved apparatus for operating electricdischarge devices is driven in re sponse to a feedback means in voltagestep-down arrangement;

FIGURE 5 is a schematic circuit diagram of the feedback driven switchshown in block form in FIGURE 4;

FIGURE 6 illustrates an oscillogram one cycle of the lamp currentprovided by the apparatus of FIGURE 4;

FIGURE 7 illustrates the lamp current waveform corresponding to aresistive value of one ohm for the shunt resistor R and 680 ohms for theresistor R of the apparatus shown in FIGURES 4 and 5;

FIGURE 8 illustrates the lamp current waveforms showing the effectproduced by changing the resistive value of the shunt resistor R from .5ohm to two ohms in the apparatus shown in FIGURES 4 and 5;

FIGURE 9 illustrates lamp current waveforms showing the effect producedby varying the tunnel diode bias current from a minus .68 milliampere toa positive .035 milliampere in the apparatus shown in FIGURES 4 and 5;

FIGURE 10 illustrates lamp current waveforms showing the effect producedby varying the resistive value of the resistor R connected in circuitwith the anode of the tunnel diode in apparatus shown in FIGURES 4 and5;

FIGURE 11 is a schematic circuit diagram of simplified feedback drivenswitching arrangement in accordance with the invention;

FIGURE 12 is a schematic circuit diagram of another embodiment of theinvention wherein the supply voltage is stepped up and the switchingmeans is driven by a freerunning driver;

FIGURE 13 is a schematic circuit diagram of the driver shown in blockform in circuit diagram or" FIGURE 12;

FIGURE 14 is an illustration of the waveform of instantaneous lampcurrent provided by the apparatus shown schematically in FIGURE 12;

FIGURE 15 is a schematic circuit diagram of one form of the inventionwherein the supply voltage is stepped up and the switching action isregulated in response to the current in an input lead of the apparatus;

FIGURE 16 is a simplified schematic circuit diagram of an apparatusembodying the invention for operating electric discharge lamps with abidirectional current in a voltage step-down arrangement;

FIGURE 17 is a schematic circuit diagram of the bidirectional switchshown in block form in the diagram of FIGURE 16;

FIGURE 18 is a schematic diagram of an apparatus embodying the inventionfor operating electric discharge lamps with a bidirectional current in avoltage step-up ara rangement;

FIGURE 19 is a schematic circuit diagram of the bidirectional switchshown in block form in FIGURE 18.

In order to provide an outline of the more detailed disclosure of myinvention, the major divisions of the remainer of this specification aresummarized under the following headings:

I. Current Level Switching Arrangement for Operating Electric DischargeDevices Providing A Voltage Step- Down.

A. Description of Apparatus Shown in FIGURES 1 and 2.

B. Discussion of the Operation of Apparatus Shown in FIGURES 1 and 2.

C. Description of Apparatus Shown in FIGURES 4 and 5.

D. Discussion of the Operation of the Apparatus Shown in FIGURES 4 and5.

E. Description of the Apparatus Shown in FIG- URE 11.

II. Current Level Switching Arrangement for Operating Electric DischargeDevices With a Step-Up in Voltage. A. Description of the Apparatus Shownin FIGURES l2 and 13. B. Discussion of the Operation of the ApparatusShown in FIGURES 12 and 13. C. Description of the Apparatus Shown inFIGURE 15. D. Discussion of the Operation of the Apparatus Shown inFIGURE 15.

III. Apparatus for Operating Electric Discharge Lamps With aBidirectional Current Employing the Improved Current Level SwitchingArrangement of the Inven tion.

A. Description of the Apparatus Shown in FIGURES 16 and 17.

B. Discussion of the Operation of the Apparatus Shown in FIGURES 16 and17.

C. Description of the Apparatu Shown in FIGURES 18 and 19.

D. Discussion of the Operation of the Apparatus Shown in FIGURES 18 and19.

D. Discussion of the Operation of the Apparatus Shown in FIGURES 18 and19.

IV. General Considerations.

As a descriptive aid, the following reference letters will be usedherein with consecutively numbered subscripts to designate variouscircuit components as indicated: C, capacitor; D, diode; L, inductor; P,primary winding; Q, transistor; R, resistor; S, secondary Winding; T,transformer; and TD, tunnel diode.

In the drawings the conventional symbols for NPN and PNP transistors areemployed. When the arrow on the emitter electrode is directed inwardlytoward the base, the symbol denotes a PNP transistor and when the arrowon the emitter electrode is directed away from the base, the symboldenotes a NPN transistor.

In the interest of simplification, auxiliary starting aid circuits havenot been shown in the schematic circuit diagrams of the illustratedexemplifications of my invention. It will be appreciated that startingaid circuits are not essential to the starting of an electric dischargelamp, and that where one is used it is generally possible to start thelamp at a lower potential level. In a well-known arrangement aconductive plate or strip is disposed in proximity to the lamp so that astarting aid potential is initially applied across an electrode and theconductive plate or strip to aid in initiating the electric discharge inthe lamp. Accordingly, it will be understood that such auxiliarystarting aid arrangements may be used in conjunction with the apparatusfor operating electric discharge lamps to be hereinafter described.

I. CURRENT LEVEL SWITCHING ARRANGEMENT FOR OPERATING ELECTRIC DISCHARGEDE- VICES WITH A VOLTAGE STEP-DOWN.

A. Description of apparatus shown in FIGURES 1 and 2.

In FIGURES 1 and 2, I have schematically illustrated an apparatus 10incorporating one form of my invention in which an electric dischargedevice, such as a fluorescent lamp 11, is operated with direct currentsupplied by a full wave bridge rectifier 12. It will be noted that aninductive element L is connected in circuit with the lamp 11 although aninductor does not impede the flow of direct current and would not beused as a ballasting element in a conventional ballast for operatingelectric discharge lamps with direct current. Generally, electricdischarge devices operated with direct current are ballasted by aresistor or other resistive means such as an incandescent lamp.

Having more specific reference now to the schematic circuit diagramillustrated in FIGURE 1, the apparatus 10 is shown enclosed in a dashedrectangle 13. The apparatus 10 is energized by connecting a pair ofinput terminal leads 14 and 15 in circuit with a suitable alternatingcurrent supply which is schematically indicated as being sinosoidal inwaveform.

In this particular embodiment of my invention, I have included a fullwave bridge rectifier 12 as a component of the apparatus 10. It will beunderstood that the bridge rectifier 12 does not necessarily have to beincluded as a component of the apparatus 10. Where a suitable D.C.source is available, electrical leads 16, 17 may be connected thepositive and negative side of the DC. source.

To provide the switching action in accordance with the invention, atransistor switch Q is coupled with a driver 18 which is shown in blockform in the diagram of FIGURE 1 and in a schematic diagram in FIGURE 2.The transistor switch Q is connected in circuit with the negativeterminal 19 of bridge rectifier 12 by electrical lead 17 and also incircuit with the lamp 11 by an output lead 21. The positive terminal 20of bridge rectifier 12 is connected by means of lead 16 with one end ofan inductor L which serves as an energy storage element, while the otherend of the inductor L is connected in circuit with lamp 11 by outputlead 22. A diode D is connected in a shunt path across inductor L andlamp 11 by means of electrical leads 23 and 24 so that when thetransistor switch Q is activated into a high impedancestate and thepower source is cut off from the lamp 11, a closed path is provided forexcursions of current from the inductor L by the circuit meansconnecting the inductor L the lamp 11, and diode D (leads 21, 22, 23 and24).

Turning now more specifically to FIGURE 2, I have illustrated therein aschematic circuit diagram of the driver 18 corresponding to the block 18in the apparatus 10 of FIGURE 1. The driver 18 is connected in circuitwith the NPN transistor switch Q by means of the electrical leads 25, 26and 27 which, as shown in both FIG- URES 1 and 2, are connected incircuit with the emitter, base and collector electrodes respectively oftransistor switch Q. A Zener diode D is connected in circuit across thetransistor Q by leads 25, 27 and 28 to protect the transistor Q fromvoltage transients. In order to reduce the delay that the transistor Qexhibits before its collector current turns off, a resistor R connectedto the lead 25 provides a path for the base leakage current.

The driver 18 used in the exemplification of the invention is an astablemultivibra-tor and supplies a rectangular pulse output across theemitter and base electrodes of the NPN transistor switch Q at apredetermined frequency as primarily determined by variable resistors RR and a timing capacitor C The timing capacitor C is connected incircuit with a DC. voltage bias supply, as for example, a positive 12volt supply,

through the resistor R and a diode D The DC. voltage applied at terminal259 provides the base drive for the transistor Q It will be seen thatthe base drive current is supplied to the transistor Q through resistorR Zener diode D diode D and the lead 26. The Zener diode D insures thatthe transistor Q is forward biased only when the voltage at the anode ofthe Zener diode D is in excess of its reverse breakdown voltage, whichin the exemplification of the invention shown in FIGURES 1 and 2 was 6.8volts. The diode D serves as a reverse blocking diode means and preventsforward current flow through the Zener diode D It will be noted that oneof the base electrodes of unijunction transistor Q is connected incircuit with the base electrode of the transistor switch Q and the otherbase electrode is connected to the electrical lead 25. When theunijunction transistor Q is in a nonconducting state, the base electrodeof the NPN transistor switch Q will be positive with respect to theemitter and thereby cause transistor Q to be switched to a low impedancestate or to a closed position or condition.

Although in the embodiment of the invention illustrated in FIGURES 1 and2 I have described a transistor as a suitable switching means, it willbe appreciated that other switching devices as well as other drivingcircuits may be employed in the practice of our invention. For example,a silicon controlled rectifier driven by a DC. chopper circuit of thetype described at pages 142-148, Figure 9.9, of the General ElectricSilicon Controlled Rectifier Manual, Second Edition 1961, may be adaptedfor use as a driven switching means in the apparatus of the invention.

B. Discussion of the operation of apparatus shown in FIGURES 1 and 2.

When a driving voltage E is applied across the leads 16, 17, a voltage Eis supplied across the output leads 21, 22 and the lamp 11. As theunidirectional switch Q is repetitively activated from a high impedancestate to a low impedance state at a predetermined frequency, the powersource is alternately connected with the lamp 11 for an interval t anddisconnected for an interval 2 1)- Referring to the oscillogram shown inFIGURE 3, it will be seen that beginning at point where the lamp currentis zero, the lamp current is building up in a positive direction for aninterval t During the interval t transistor switch Q is in a lowimpedance state and the output of the full wave bridge 12 is connectedin circuit with lamp 11. It will be understood that when transistorswitch Q is in a low impedance state, diode D is in a blocking statesince the voltage at its cathode is positive with respect to the voltageat the anode. Further, the polarity of the voltage across the inductor Lis such that its left end as viewed in FIGURE 1 is positive with respectto its right end.

At the end of the interval t transistor switch Q is activated to a highimpedance state and the power to the lamp 11 from the source isinterrupted during the interval (t -t At the instant that the drivenswitch Q is opened, the magnetic field in the inductor L begins tocollapse. A counter is induced in inductor L that opposes the decline ofthe lamp current. Thus, during the interval (2 4 the polarity of thevoltage across the inductor L is such that the left end of the inductorL is negative with respect to the right end thereof. The diode D in theshunt path of lamp L is now forward biased. The electrical energy storedin the inductor L is now supplied to the lamp in the form of a decayingcurrent. The decaying current supplied to the lamp 11 during theinterval (t -t is represented in FIG- URE 3 by portion a-b of the lampcurrent waveform.

At point b, transistor switch Q is again driven to its low impedancestate and the output of the bridge 12 is connected in circuit with lamp11. For another interval t electrical energy is supplied to the inductorL and to lamp 11 from the power source. When the instantaneous lampcurrent reaches point c at the end of a second interval t transistorswitch Q is driven to its open position or high impedance state.

It will be apparent that the portions of the lamp current waveform o-a,bc and d-e represent excursions of energy from the power source whilethe portions of the lamp current waveform a-b, c-d and e]" representexcursions of energy from the storage element or inductor L Astransistor switch Q is successively opened and closed, controlledexcursions of energy are alternately supplied from the storage element Land the power source to maintain and control the electric discharge oflamp 11.

If we assume the electrical discharge lamp 11 is essentially a resistiveload, the average voltage E across the lamp will be approximately equalto the product of ratio t /t and the average value of the drivingvoltage E. It will be evident from this relationship that the lampvoltage E can be readily varied by the conduction and nonconductionintervals t and (t t of the transistor switch Q The intervals t and (r 4are controlled by the driver 18 which activates the transistor switch Qat a predetermined frequency. The interval t is essentially determinedby the duration of the positive pulse applied across the base andemitter electrodes of the transistor Q The duration or width of thepositive pulse is determined by the nonconduction interval ofunijunction transistor Q It will be understood that the unijunctiontransistor Q becomes forward biased, i.e., switched on, when the voltagecharge on the timing capacitor C reaches the peak point voltage of theunijunction transistor Q When the unijunction transistor Q is switchedon, the anode of diode D is clamped to lead 25, and diode D is reverselybiased. The capacitor C then discharges through the variable resistor Rand at a predetermined point transistor Q is turned off. The amount ofresistance intro duced in the path of this discharge controls theturnoff point of the unijunction transistor Q and consequently theduration of the interval (l t or the. off time of the transistor switchQ Thus, if the variable resistor R is set to provide a high resistance,the point at which the potential at the cathode of the diode D becomesapproximately equal to the junction potential of the unijunctiontransistor Q is delayed. When these potentials are about equal, thediode D conducts current, and the emitter current decreases causing theunijunction transistor Q to cut off, thereby causing the bias voltage tobe applied across the transistor switch Q When diode D is in aconducting state, charging cur rent is supplied to the timing capacitorC The charging rate is controlled by the variable resistor R whicheffectively provides a control for the duration of the interval t or theon time of the transistor switch Q Increasing the amount of theresistance in the path of the charging current will, of course, delaythe point at which the charge. on the timing capacitor C reaches thepeak point voltage of the unijunction transistor Q and consequentlyprolong the interval t When the charge on the capacitor C reaches thepeak point voltage, unijunctlon transistor Q is switched on again toremove the positive pulse applied across the base and emitter electrodesof the transistor switch Q The timing capacitor C then begins todischarge again through the resistor R and the cycle repeats itself.

From the foregoing description of the operation of the apparatus shownin FIGURES l and 2, it will be apparent that the switching intervals ofthe transistor switch Q can be readily varied by selecting suitablevalues for the resistors R and R to provide the switching action tocontrol the operation of lamp 11 by repetitively switching the lampcurrent between high and low current levels. This switching action notonly produces a step-down of the supply voltage required for the lamp 11but also limits the lamp current.

9 C. Description of apparatus shown in FIGURES 4 and An importantadvantage of the apparatus of the invention resides in its adaptabilityto regulation by means of a feedback means. The on and off intervals, tand (z -t of the current level switching means can be readily controlledduring operation by feedback arrangements which sense the lamp current,the light intensity of the electric discharge device, lamp voltage, lamppower, lamp hot spot temperature, or combinations of these and otherquantities such as line voltage, ambient temperature and ambientillumination levels.

In FIGURE 4, I have illustrated a specific embodiment of my invention inwhich a transistor switch Q is driven in response to the feedback ofcurrent from the lamp circuit. The apparatus 30 illustrated in FIG- URE4 is generally similar in arrangement to the apparatus shown in FIGURE 1except that the transistor switch Q is driven by a feedback means and isnot a free running switch.

The apparatus 30 controls the operation of an electric discharge lamp 11and is shown enclosed in a dashed rectangle 31. For the purpose ofenergizing the apparatus 30, a pair of input terminal leads 32 and 33are brought out externally for connection to a suitable D.C. sourcewhich is not shown. The DC. source may be a filtered or unfilteredrectified alternating current supply, a battery or other D.C. source.

An inductor L serves as the storage element and is connected in the lampcircuit by output leads 34 and 35. A diode D is connected in a shuntacross lamp 11, inductor L and resistor R to provide a path for thecurrent supplied from the inductor L when the transistor switch Q isswitched to the open position. Resistor R is connected in circuit withoutput lead 35 at one end and at the other end with an electrical lead36 joined to the emitter of the transistor switch Q The voltage dropacross the shunt resistor R is coupled to a driver 37 by means of a pairof feedback leads 38 and 39. A pair of leads 40, 41 are provided forconnection to a suitable alternating supply to energize the driver 37. AZener diode D is connected in shunt with the transistor switch Q, toprotect it from transient voltages.

In FIGURE 5, I have illustrated a schematic circuit diagram of thedriver 37 corresponding to the driver 37 represented in block form inthe apparatus 30 of FIG- URE 4. The connecting leads 38, 39, 40 and 41as shown in FIGURE 5 correspond to the connections of the schematiccircuit diagram of FIGURE 4 which are identified by the same referencenumerals. It will be noted that I have included in FIGURE 5 thetransistor switch Q and its connections.

The driver 37 is shown enclosed in a dashed rectangle and includes a PNPtransistor Q.,, a NPN transistor Q and a tunnel diode TD that controlthe switching action of the transistor switch Q, as will hereinafter bemore fully described. Resistors R R R and R maintain proper biasingconditions for the transistors Q Q and Q A resistor R connected in thefeedback lead 39 limits the feedback current.

The switch driver arrangement illustrated in FIGURE 5 is more fullydescribed and claimed in US. Patent No. 3,151,289 granted to Theodore R.Harpley and assigned to the same asignee as the present invention. Analternative switch driver arrangement which may be used in apparatus 30is shown in FIGURE 11 and will be more fully discussed in connectionwith the description of FIGURE 11.

The base drive for the transistor Q of the driver 37 and the driventransistor switch Q is provided by a bias or driver power supply whichincludes input leads 40, 41, a full wave bridge rectifier 42, and acapacitor C A resistor R limits the current output of the driver powersupply. A Zener diode D clips the positive voltage at lead 43 to itsreverse breakdown voltage (6.8

volts), and a positive bias current is supplied at lead 43. Also, apositive bias equal in magnitude to the sum of the forward voltage dropof diode D and the reverse breakdown voltage of Zener diode D isprovided at lead 44. A second Zener diode D connected in circuit withthe negative terminal of the bridge 42 regulates the negative voltageand voltage equal to its reverse breakdown voltage (6.8 volts) isprovided at lead 45 for the PNP transistor swtich Q The tunnel diode TDconnected across the feedback leads 38 and 39 of the driver 37 is a twoterminal semiconductor device having a single PN junction. The P- layeris referred to herein as the anode and is shown schematically by thevertical line while the N-layer is referred to as the cathode and isshown schematically as an arcuate portion joined to the vertical ortransverse line. When a forward voltage less than the peak point voltageis applied across the tunnel diode TD it will exhibit a low resistanceand may be considered as being in a low impedance state. For a range ofintermediate values of the forward voltage, the tunnel diode TD, ischaracterized by a negative conductance. When the forward voltageexceeds the valley point voltage, the tunnel diode TD has acharacteristic similar to the forward characteristic of a conventionaldiode and may be considered as being in a high impedance or high voltagestate. The characteristics of tunnel diodes of the type that may be usedin the practice of our invention are more fully described in Chapter 2of the General Electric Tunnel Diode Manual, First Edition, 1961.

D. Discussion of operation of the embodiment shown in FIGURES 4 and 5When the input terminal leads 32, 33 of apparatus 30 are connected incircuit with a DC. voltage source, of the polarity indicated on thedrawing an open circuit condition in effect exists across the outputleads 34 and 35, since the electric discharge lamp 11 presents asubstantially infinite impedance before the electric discharge isinitiated. During the open circuit condition there is no current flowthrough the shunt resistor R and consequently the voltage drop acrossthe resistor R is at a Zero level. The driver 37 holds the transistorswitch Q in a low impedance state during this open circuit condition. Itwill be noted that during the open circuit condition the supply voltageE is approximately equal to the voltage E applied across the electricdischarge lamp 11 and must be sufiicient to start lamp 11.

When the electric discharge is initiated, current flows through the lamp11 and through the shunt resistor R The voltage drop across the shuntresistor R increases proportionally with the current and when thecurrent reaches a first level, the transistor switch Q is driven to itsopen position or high impedance state. The power source is in effectdisconnected from the lamp circuit. This interruption of power in thelamp circuit causes a counter to be induced in the inductor L which isnow a reverse polarity. This reversal of polarity results in a forwardbias across the diode D Diode D conducts, and a path is provided throughdiode D for an excursion of energy in the form of a decaying currentfrom the inductor L When this current declines to a second predeterminedlevel, the transistor switch Q is driven to a closed position, andcurrent is supplied to the lamp 11 from the power source. During thisswitching mode, energy is also being stored in the inductor L It will beseen from the lamp current waveform shown in FIGURE 6 that as a resultof the repetitive switching action of transistor switch Q energyexcursions from the power source and inductor L alternately occurbetween the two predetermined levels. Since the energy level of thesupply source is greater than that required to maintain the electricdischarge in lamp 11, the energy supplied to lamp 11 from the sourceseeks to rise to this level but is chopped at a predetermined point bythe 1; l. transistor switch Q As will be seen in the waveform shown inFIGURE 6, the portions a-b, c-d represent excursions of energy from theinductor L and the portions b-c, d-e represent excursions of energysupplied from the power source. The valleys and the peaks of thesecurrent or energy excursions are controlled by the feedback to thedriver 37 which actuates the transistor switch Q Since an unfilteredrectified direct current supply was applied across input leads 32, 33 inthe illustrated embodiment of the invention of FIGURES 4 and 5, it willbe appreciated that the lamp current waveform is cyclical. At the end ofeach cycle the lamp current decreases to a value below the minimum leveleven though the transistor switch Q is held in the closed position untilthe current builds up in the succeeding half cycle.

Having more specific reference now to FIGURE 5, the operation of thedriver 37 will now be more fully described. When the apparatus 30 isinitially energized and no current flows through the shunt resistor Rthe transistor switch Q; is in a conducting state because base drive isapplied at the base electrode of transistor switch Q from lead 45 of thebias supply through resistors R and R During this initial period,transistors Q and Q are in a non-conducting state. After the electricdischarge in lamp 11 is stated, current builds up in the feedback lead39. When this current reaches the peak point value of the tunnel diodeTD it switches from a low impedance state to a high impedance state. Itwill be noted that when the tunnel diode TD is triggered into a highimpedance state, the base electrode of the transistor Q is more positivewith respect to the emitter electrode than in the previous low impedancestate of tunnel diode TD and as a result transistor Q, is switched on.

When transistor Q conducts, the base of transistor Q; is returnedthrough resistor R to the negative side of the Zener diode D of the biassupply. Since the emitter electrode of transistor Q; is connected tolead 43 of the bias supply which supplies a positive bias (6.8 volts),the base electrode of transistor Q, is negative with respect to theemitter, and transistor Q conducts. The positive bias at lead 46 is nowapplied at the base electrode of transistor Q and the base of transistorQ becomes positive with respect to its emitter. Transistor Q is turnedoff, and the direct current supply to the apparatus 30 is switched off.

When transistor Q is turned off, the power supply is essentiallydisconnected from the lamp circuit and the lamp 11 is energized by adecaying current supplied from the inductor L The feedback currentthrough resistor R now proportionally decreases with the lamp current,and when the voltage across TD; reaches the valley point value of thetunnel diode TD the tunnel diode TD is reset to its low impedance state.When the tunnel diode TD reverts to its low impedance state, the base ofthe transistor Q is no longer sufficiently positive with respect to theemitter, and transistor Q returns to a nonconducting state and turns offtransistor Q With transistor Q switched off, the negative bias supply atlead 45 is applied at the base of transistor Q through resistors R and RTherefore, transistor Q is switched into a conducting state and thepower source is again connected with the electric discharge lamp 11.

The apparatus 30 illustrated schematically in FIG- URES 4 and 5 wasconstructed and satisfactorily operated two 30 watt fluorescent lamps inparallel and was capable of operating eight such lamps. The main supplyvoltage was obtained from a 120 volt, 60 cycle supply and was rectifiedby a full wave bridge rectifier. The unfiltered output of the bridgerectifier was applied across terminals 32 and 33. The alternating powersupply was stepped down by a small transformer to provide a 26 voltsupply at terminals 40 and 41 of driver 37. As is well-known in the art,a starting aid potential for lamp 11 was provided by placing aconductive plate in proximity to the lamp Ill so that the lamp 11 wasdisposed in capacitive relationship therewith. Also, a small filamenttransformer was used to supply heating current to the lamp filamentssince a hot cathode type of lamp was used.

Referring now to FIGURES 4 and 5, the following circuit components wereused in the illustrative exemplification of the invention and are givenby way of example:

Bridge rectifier 42 Mallory FW 600.

Inductor L 50 millihenries.

Shunt resistor R 1 ohm.

Resistor R 10 ohms.

Resistor R 1000 ohms.

Resistor R ohms.

Resistor R 1000 ohms.

Resistor R 1500 ohms.

Resistor R 100 ohms.

Transistor Q -.Bl0133 Bendix development type.

Transistor Q 2N241A.

Transistor Q 2N636A.

Tunnel diode "ED .IN2939.

Zener diode D -.IN3051.

Zener diode D -.IN3016B, 6.8 volts,

Zener diode D -IN3016B, 6.8 volts.

Zener diode D .IN1695.

The adaptability of the current level switching arrangement of theinvention to feedback type of regulation is graphically demonstrated bythe lamp current waveforms shown in FIGURES 7, 8, 9 and 10, which showthe lamp current wavefonms obtained for various values of differentcircuit parameters. In FIGURE 7 I have illustrated the lamp currentwaveform as observed on a cathode ray oscilloscope when the resistanceof resistor R was one ohm and the resistance of the the resistor R was680 ohms, which served as a basis for comparison. With these values,normal light output was obtained with a lamp current of 260 milliamperesD.C. and the frequency of switching was approximately 300 cycles persecond. When the resistive value of the shunt resistor R was increasedto 200 ohms, the lamp current was reduced to approximately 2.6milliamperes and a dimming ratio of approximately 100 to 1 was readilyestablished. The frequency of switching at the low dimming level wasapproximately 30 kilocycles.

From the lamp current waveforms shown in FIGURE 8, it will be seen thatsmall increases in the resistive value of the shunt resistor R have theeffect of substantially increasing the switching frequency. Also, itwill be seen that for a resistive value of .5 ohm, only one excursion ofenergy was produced in each cycle of the current waveform. However, whenthe resistive value was increased to two ohms, five successiveexcursions of energy from the inductor L were produced in each cycle ofthe lamp current waveform.

It will be apparent fro-m the lamp current waveforms shown in FIGURE 9that the lamp current can be readily varied by changing the bias appliedto the tunnel diode TD and also by varying the resistive value of theresistor R connected in the feedback lead 39. As is shown in F lG-URE10, when the resistive value of resistor R is decreased from 1500 ohmsto 330 ohms, the magnitude of the lamp current decreases and theswitching frequency also increases.

If it is desired to dim an electric discharge lamp over a wide range ofdimming levels, this can be easily accomplished by varying the resistivevalue of the shunt resistor R and the resistor R by varying the tunneldiode bias current separate-1y or in combination with the resistors R RFor example, a value of bias current at some predetermined negativevalue within the limits imposed by tunnel diode dissipation may beselected to provide maximum light output at a peak current to valleycurrent ratio of approximately 1.05 to 1. Where an electric dischargelamp is dimmed by varying one or more of the aforementioned circuitparameters, it will be understood that the open circuit voltageavailable to start the lamp is not affected. Consequently, the lamp isreadily maintained in conduction at low dimming levels.

Referring again to the lamp current waveforms in FIGURES 7, 8, 9 and 10,it wil be observed that the last peak of the cycle is not sharplydefined like the peaks which occur earlier in the cycle. This is due tothe fact that at the end of each half cycle the main supply voltage isfalling off rapidly.

Although a rectified single phase alternating supply was used in theexemplification of the invention described in FIGURES 4 and 5 to operatethe apparatus 30, a rectified three phase alternating supply can beadvantageously employed in the practice of the invention. When a threephase rectified alternating power supply was used, it was found that thepeak voltage to average voltage ratio could be reduced to approximately1.05 and the switching mode of current control is continuous.

Since the peak value of the rectified voltage in a single phase systemis approximately twice that of the average value required for continuousconduction of the electric discharge lamp 11, the semiconductor devicesused in such a system must be capable of handling the peak voltage. Anadvantage resulting from the use of a rectified three phase power supplyis that it may be possible to employ semiconductor devices having alesser power rating than those that would be used in a rectified singlephase system.

Although in this illustrated embodiment of the invention the switchingmeans was driven in response to the current flow in the lamp circuit, itwill be apparent that the feedback arrangement can be readily used tosense other quantities such as the light output of the electricdischarge device using a photoelectric cell to sense the light output ofthe lamp. A combined light and current sensing arrangement provides thedistinct advantage that a nearly constant light output can be maintainedfor the life of the electric discharge lamp. Accordingly, it will beevident that the present invention is readily adaptable to control byfeedback means.

E. Description of apparatus shown in FIGURE 11 In the schematic circuitdiagram shown in FIGURE 11, I have illustrated an apparatus '50 whichembodies an alternative switch driving arrangement and which isgenerally similar to the apparatus 30 shown in FIGURE 4. As shown inFIGURE 11, the components of apparatus 50 are enclosed in a dashedrectangle 51. To show the correspondence between apparatus 50 of FIGURE11 and apparatus 30 of FIGURE 4, I have employed the same referencesymbols to identify the corresponding parts thereof.

The current level switching function in apparatus 50 is performed by adriven PN'P junction transistor Q connected in circuit with a terminallead 33 adapted for connection to the negative side of the DC. source.An inductor L serves as the energy storage element and is connected incircuit with output lead 35. A shunt resistor R serves as a sensingelement of the feedback means and is joined in circuit with lamp 11 byan output lead 34 at one end and at the other end by the input terminallead 32 provided for connection to the positive side of the DO. powersupply. Diode D is connected in a shunt path across lamp 11 and inductorL for the excursion of energy from the inductor L when the transistorswitch Q is switched to the open position. Zener diode D connectedacross transistor switch Q protects the transistor switch Q againstdamage from excessive transient voltages.

It will be noted that in the switch driving arrangement shown in FIGURE11 the feedback means, which includes a tunnel diode TD resistors R Rand R are coupled with the switch driver means by a pulse transformer Thaving a primary winding P and a secondary winding S The switch drivermeans includes a second tunnel diode TD a capacitor C a resistor R and apositive and negative driver supply terminals 52 and 49. The currentprovided through resistors R and R was suflicient to bias tunnel diodeTD slightly below its peak point value. The capacitor C provides D.C.isolation of the secondary winding S and pulse coupling with the tunneldiode T13 The resistor R was required to prevent ringing in the loopwhich includes the primary winding P and the tunnel diode TD Since inthis switch driving arrangement, transistor switch Q is normally in anopen position, the apparatus 50 is not self-starting. Accordingly, ameans for initially switching the transistor switch Q into a closedposition is provided which includes a starting switch 54, a capacitor Cresistors R R and a terminal 49 for connection to a negative biassupply.

The closing of switch 54 causes the resistor R to be short-edmomentarily all-owing sufiicient bias current to flow into tunnel diodeTD to cause it to flip to the high voltage state. When capacitor C ischarged, the short circuiting of resistor R ceases. It will beappreciated that the capacitor 0., can be eliminated where the closinginterval of switch 54 can be controlled. Resistor R allows capacitor Cto discharge after switch 54 is released.

The apparatus 50 is energized by connecting the input terminals 3 2, 33to the positive and negative sides, respectively, of a DC. source. Also,the terminals 49, 52 are connected to a suitable DC. bias supply.

The operation of the apparatus 50 is initiated by actuating switch 54.As was previously described, the closing of switch 54 causes theresistor R to be shorted momentarily allowing s-ufiicient bias currentto flow through resistor R to tunnel diode TD and switch it to a highvoltage state. Consequently, transistor switch Q is switched to theclosed position. When transistor switch Q is in the closed position, thepower source across input terminal leads 32, 33 is connected in circuitwith the lamp 1 1. The path of current flow is through the positiveinput terminal lead 32, the shunt resistor R the output lead 34, thelamp 111, the output lead 35, the inductor L the transistor switch Q andthe negative input terminal lead 33 back to the power source.

Since during this switching mode energy is being stored in the inductorL the current through the shunt resistor R is building up in magnitudeand causing a proportionally increasing voltage drop across the shuntresistor R The polarity of this voltage is such that the left end of theshunt resistor R is positive with respect to the right end. Aproportionally increasing current is supplied to the feedback resistor Rand to the loop which includes the tunnel diode TD the resistor R andthe primary winding P When the current through the tunnel diode TDreaches the peak point value, tunnel diode TD is switched to a highvoltage state. This causes a sharp increase in the impedance of thetunnel diode TD and a change in the current flow through the resistor Rand the primary winding P As a result, a positive pulse is inducedacross the secondary winding S tunnel diode TD is reset to its lowvoltage state and transistor switch Q is switched to its open position.

When transistor switch Q, is in the open position, lamp 1'1 is energizedwith a decaying current, since the main power source is disconnectedfrom lamp 11. During this switching mode lamp 11 is energized by therelease of the energy stored in the magnetic field of the inductor LAccordingly, the instantaneous current through the shunt resistor R isdecreasing in magnitude, and the voltage drop across the shunt re-sistorR is thereby proportionally decreasing. The current flow through thefeedback resistor R and through the tunnel diode loop alsocorrespondingly decreases. When the current through the tunnel diode TDdrops below its valley point value, the

tunnel diode switches to its low voltage state thereby effecting a sharpdiversion from the primary winding P and causing a negative pulse to beinduced across the secondary winding S This negative pulse causes thetransistor switch Q to be closed thereby again connecting the powersource in circuit with the lamp 11. This switching action continuesrepetitively so long as the voltage applied across the shunt resistor Ris sufiicient to actuate the tunnel diode TD II. CURRENT LEVEL SWITCHINGARRANGE- MENT FOR OPERATING ELECTRIC DISCHARGE DEVICES WITH A STEP-UP INVOLTAGE.

A. Description of the apparatus shown in FIGURES 12 and 13 In thepreceding section, I have described embodiments of the inventionemploying the concept of operating electric discharge lamps by arepetitive switching action wherein the voltage output of the apparatuswas less than the voltage applied at its input. To further illustrate.the versatility of the invention, -I shall now describe additionalembodiments of the invention wherein an output voltage In other words, astep-up in the voltage is achieved in I contrast to the voltage step-down provided by the previously described embodiments of my invention.

Referring now to the schematic circuit diagram shown in FIGURE 12, Ihave illustrated therein an apparatus 60, the components of which areenclosed in dashed rectangle 61. The apparatus 60 is energized byconnecting a pair of input terminal leads 62 and 63 in circuit with thepositive and negative side of a DC. power source. The direct currentpower source may be a filtered or unfiltered rectified alternatingcurrent supply, a battery or other suitable DC power source. The voltageoutput of the apparatus 60 is applied by output leads 64, 65 across apair of serially connected electric discharge lamps 1 and 2.

Although commercially available electric discharge lamps have been usedin the exemplifications of the invention, it will be appreciated thatthe presently proposed concept of operating and ballasting electricdischarge devices makes it attractive and practicable to developelectric discharge lamps for use in conjunction with the improvedapparatus of the present invention. Present day electric fluorescentlamps have been designed as high voltage, low current type of devices.The apparatus in accordance with the invention renders electricdischarge lamps designed for higher current and lower voltage operationparticularly desirable in view of the current characteristics of solidstate devices.

Continuing with the description of the physical arrangement of theapparatus 60 as shown in FIGURE 12, it will be seen that a firstinductor L a diode D and a second inductor L; are connected in seriescircuit with the lamps 1 and 2. An NPN junction transistor switch Qserves as the switching means and is driven by a driver 66, which isconnected to the base electrode of transistor switch Q by lead 67 and incircuit with the negative side of the power source by lead 68 and inputlead 63. It will be noted that in this exemplification of the inventionwhen the transistor switch Q is driven to the closed position, a pathfor the current from the power source is provided which shunts the lamps1 and 2.

Although a transistor switch was used in the illustrated embodiment, itwill be understood that other switches capable of being repetitivelyswitched from a low impedance state to a high impedance state a arelatively fast rate can be employed. For example, a silicon controlledrectifier switch driven by a multivibrator circuit may be used toprovide the switching action across input leads 62, 63 required tointermittently disconnect the power source from the lamps 1 and 2.

A capacitor C connected in circuit across inductor L and the seriallyected lamps 1 and 2 serves as a first energy storage element and theinductor L serves as a second energy storage element as will hereinafterbe more fully described in the discussion of the operation of theapparatus 60. The inductor L serves to control the discharge fromcapacitor C Diode D presents a high impedance between the source and thecapacitor C to prevent the discharge of energy stored in the capacitor Cto the power source and through the transistor switch Q when it is inthe closed position. Thus, the energy stored in capacitor C isdischarged through the lamps 1 and 2 and the inductor L connected inseries with the lamps.

Turning now to FIGURE 13, I have shown therein a schematic circuitdiagram of the switch driver 66 used in the apparatus 60 of FIGURE 12.In this exemplification of the invention, I have employed a rectangularwave oscillator as the driver 66. The rectangular wave output of thedriver 66 is applied across the base and emitter electrodes of thetransistor switch Q When a positive pulse appears at output lead 67, thetransistor switch Q, is turned on and is turned off when the drivecurrent is diverted through transistor Q Terminal lead 69 is providedfor connection to a positive 10 volt supply through a resistor R Inorder to speed the turn-off of transistor Q a resistor R is connected incircuit with the base electrode of transistor switch Q and in circuitwith the negative side of the main power supply by means of lead 68. Itwill be understood that when tunnel diode TD is switched to its highvoltage state, transistor Q will conduct. In the exemplification of theinvention, the voltage drop across the collector and emitter electrodesof transistor Q7 was approximately one volt and might be sulficient todrive the base-emitter junction of transistor Q To prevent this voltagefrom driving the base-emitter junction of transistor Q to a conductingstate, a silicon diode D is con nected in series with the base-emitterjunction of transistor Q and in effect functions as a low voltage Zenerdiode. Thus, it insures that when transistor Q conducts, the basecurrent drive is effectively diverted from transistor switch Q Thedriver 66 includes a tunnel diode TD connected across the base andemitter electrodes of a transistor Q and a charging and dischargingcircuit comprised of a capacitor C and resistors R R and R It will benoted that when tunnel diode TD is in a low voltage state, transistor Qis turned off, and transistor switch Q; is turned on since the drivercurrent is supplied to the base electrode of the transistor switch Qthrough resistor R and diode D When tunnel diode TD is in a high voltagestate, the base electrode of transistor Q becomes more positive withrespect to its emitter and it is switched to a closed position therebyclamping the collector to essentially terminal 68 and diverting the basedrive current from transistor Q through transistor Q turning transistorQ oif.

B. Discussion of the operation of the apparatus shown in FIGURES 12 and13 Having reference to the circuit diagrams shown in FIGURES 12 and 13,the operation of the apparatus 60 will now be more fully described. Theoperation of apparatus 60 is initiated by connecting terminal leads 62,63 to the positive and negative sides respectively of a suitable directcurrent source such as a rectified alternating supply. A positive DC.voltage supply is also applied at terminal 69 to energize the driver 66.

It will be understood that when the transistor switch Q is closed,energy is stored in the magnetic field of inductor L When the transistorswitch Q,- is opened, the interruption causes a reversal in the polarityof the voltage across inductor L and the voltage across the inductor Lis in an additive or boosting relation with respect to the sourcevoltage. This combined voltage is applied across capacitor C Astransistor Q; is suc- '17 cessively switched during the lamp opencircuit condition, the voltage across C builds up until it reaches alevel where it is suflicient to start the electric discharge in theserially connected lamps 1 and 2. Once the electric discharge isinitiated, the lamps 1 and 2 conduct current.

Referring to the lamp current waveform shown in FIGURE 14, it will beseen that the lamps 1 and 2 are operated by a lamp current that ischaracterized by a high frequency ripple. This ripple in the lampcurrent is produced by the repetitive switching action of transistorswitch Q which causes alternate excursions of energy from the capacitorC and from the inductor L and the power source.

Assuming that t represents the interval when the transistor switch Q; isclosed, it will be noted from the lamp current waveform shown in FIGURE14 that during this interval the current is decaying. Also, during theinterval t the voltage across the inductor L is such that the left endof the inductor L as seen in FIGURE 12, is positive with respect to theright end and energy is being stored in the magnetic field of theinductor L and the supply current is shunted through the transistorswitch Q back to the supply. Continuity of the supply of the current tothe lamps 1, 2 during the interval f is maintained by the discharge ofcurrent from the capacitor C At the end of the interval t the transistorswitch Q is driven to an open position. The resultant interruption ofcurrent flow results in a reversal of the polarity of the voltage acrossL Taking t as the interval that transistor switch Q remains open, thevoltage across the inductor L during this interval is in additiverelationship to the supply voltage and this combined voltage isessentially applied across lamps 1 and 2 in series with the inductor Land across the capacitor C Current from the D.C. supply applied acrossinput leads 62, 63 now flows through the inductor L diode D the parallelbranch which includes the capacitor C the parallel branch which includesthe inductor L and the lamps 1, 2, and to the negative side of the D.C.supply. During the interval t energy is being supplied from the powersupply and the inductor L and is being stored in the capacitor C At theend of the interval t the transistor switch Q is driven to a closedposition, and the switching cycle repeats itself. Energy is again storedin the magnetic field of the inductor L and the capacitor C dischargesto provide an excursion of current to maintain the conduction of thelamps 1 and 2.

The voltage build-up across output leads 64, 65 is a function of therelation between the stored energy and the impedance of lamps 1 and 2.If the lamps 1, 2 dissipate energy quickly, as the lamps 1, 2 will dowhen operating, the energy stored in the inductor L is relatively small,and the voltage build-up across the lamps 1, 2 is small. However, if thelamps 1, 2 do not dissipate the energy supplied across output leads 64,65, as is the case during the open circuit condition, the voltagecontinues to rise until the lamps 1, 2 are ignited and dissipate energy.

Referring now more specifically to the schematic circuit diagram of thedriver 66 shown in FIGURE 13, the interval t is determined by theduration or width of the positive pulses applied at the base electrodeof transistor switch Q The off time of these positive pulses determinesthe interval t The duration and off time of the positive pulses may bevaried by adjusting the parameters of the charging circuit of the driver66. Thus, the pulse output can be varied for example, by adjusting theresistive value of the resistors R R and R or by changing thecapacitance of the capacitor C When a positive pulse is provided at thebase electrode of transistor switch Q transistor switch Q; is in a lowimpedance state, transistor Q is in a blocking state, capacitor C ischarged through resistors R and R and tunnel diode TD is in a lowvoltage state. During this period, the current through resistor R isbuilding up and when it reaches the peak point value, the tunnel diodeTD. switches to its high'voltage state. Transistor Q, is driven tosaturation by the base current through the resistor R and clamps thepositive driver supply voltage t) the negative side of the power supplythrough lead 68 rausing the transistor Q; to be turned off. Capacitor Cthen discharges through the resistors R and R When the current at theanode of the tunnel diode TD falls below the valley point value, tunneldiode TD, is switched to its low voltage state, and one cycle of therectangular wave output is completed.

From the foregoing description of the operation of the apparatus shownin FIGURES 12 and 13, it will be apparent that energy from the powersource is repetitively connected and disconnected from the lamps 1 and 2by the transistor switch Q It: will be noted that in each switching modeof the apparatus 60 energy is stored in a storage element and isreleased from the storage element in the subsequent switching mode.During the switching mode corresponding to the open position of thetransistor switch Q energy is released from the inductor L and is beingstored in capacitor C and dissipated in the lamp load. In the succeedingswitching mode corresponding to the closed position of the transistorswitch Q energy is stored in the inductor L and released from capacitorC In this manner, the apparatus 60 provides an operating voltage forlamps 1, 2 that is greater than supply voltage and controls theoperation of the lamps 1, 2.

C. Description of the apparatus shown in FIGURE 15 Referring now to theschematic circuit diagram shown in FIGURE 15, I have illustrated thereina voltage step-up arrangement in accordance with my invention wherein aswitching means, a PNP junction transistor Q; is driven in response tovariations in the supply current as sensed by a feedback means whichsupplies a feedback signal to activatethe switch driver circuit. It willbe noted that the basic circuit configuration is similar to that shownin FIG- URES 12 and 13, except for the switch driver circuit which isnot a free running oscillator but a feedback controlled oscillator.

The apparatus shown in FIGURE 15 is generally identified by thereference numeral 7 0 and is adapted for operation from a D.C. voltagesource by connecting input terminal eads 71 and 72 to the negative andpositive side of the source. The output current of the apparatus issupplied to a pair of serially connected electric discharge lamps 1 and2 by means of output leads 73 and 74. Where hot cathode fluorescentlamps are employed, it will, of course, be understood that a heatingcurrent must be supplied to the cathodes of the lamps, as for example,by a small filament transformer (not shown).

The transistor switch Q; is connected in a shunt path across the lamps1, 2 so that when the transistor switch Q; is driven to a closedposition, the supply current shunts lamps 1 and 2 and when thetransistor switch Q; is driven to an open position, the D.C. source isconnected in circuit with the lamps 1, 2. In other words, as thetransistor switch Q is activated, the power source is switched in andout of the lamp circuit.

An inductor L is connected in circuit with the input terminal lead 71provided for connection to the negative side of the power supply andserves as a first energy storage element in accordance with theinvention. A capacitor C serves as a second energy storage element. Tocontrol the discharge of the capacitor C an inductor L is connected inthe discharge path of the capacitor C as shown in FIGURE 15. Diode Dprevents capacitor G; from dis charging through the transistor switch Qwhen it is switched to the closed position.

Transistor switch Q is repetitively switched on and oft by a drivercircuit means which senses the current in input lead 72. The drivercircuit means includes a PNP transistor Q which has its emitter andcollector electrodes con- 119 nected in cincuit across input terminalleads 71 and 72. The bias current to the base electrode of thetransistor switch Q; is limited by a resistor R A tunnel diode TD isconnected across the base and emitter electrodes of transistor Q andserves to detect the level of the current feedback. Resistors R and Rcontrol the bias conditions of the tunnel diode T13 and the transistor QA resistor R connected with the base electrode of transistor Q providesa path through which the junction displacement current may be dischargedwhen the transistor Q is switched open. A resistor R shunts current intothe circuit branch which includes the resistor R and tunnel diode TD Adiode D is connected at the base electrode of transistor Q and preventsthe voltage drop across the emitter junction of transistor Q when itconducts from driving the base emitter junction of the transistor switchQ; to a conducting state.

D. Discussion the operation 0 the apparatus shown in FIGURE Operation ofthe apparatus 7 0 is initiated by applying a DC. source across the inputterminal leads 71 and '72. The transistor switch Q; is repetitivelyopened and closed for intervals that are determined by the rate at whichthe tunnel diode TD is switched to a high voltage state and reset to alow voltage state. When tunnel diode TD is in a low voltage state, thetransistor O is switched off, since its base electrode is clamped to theemitter and positive side of the power source through input lead 72.Transistor Q; is driven to saturation by the current flow through a pathwhich includes the terminal lead 72, the emitter and base electrode oftransistor Q diode D the resistor R and terminal lead 71. Accordingly,transistor Q; is switched into a closed position.

During this switching mode, the path of current flow is from inputterminal lead 72, through the shunt resistor R the transistor switch Qthe inductor L and to the input terminal lead 71. Thus, energy is beingstored in the magnetic field of the inductor L and the current isbuilding up causing the voltage drop across the shunt resistor R toincrease. Also, the current through resistor R is increasing, and whenthis current reaches the peak point value the tunnel diode TD isswitched to its high voltage state. The base electrode of transistor Q,is now negative with respect to the emitter, transistor Q conducts anddiverts the current from the base electrode of transistor Q causing itto turn oif.

At the instant that transistor switch Q; is turned ofi, the polarity ofthe voltage across the inductor L reverses so that it is in additiverelationship to the source voltage. Hence, inductor L discharges itsstored energy and steps up the source voltage. The current now suppliedto lamps 1, 2 decays. The feedback current to tunnel diode TD alsoproportionally decays, and when the voltage across the tunnel diode TDdrops to its valley point value, the tunnel diode TD is reset to a lowvoltage state, thereby causing the transistor O to be switched off andtransistor switch Q; to be switched on. Electric energy is again storedin the inductor L and the cycle continues to repeat itself. As theswitching cycle repeats itself, the voltage across the capacitor Cbuilds up until the lamps 1, 2 are started (it is not necessary thatthey be allowed to go out in each cycle of operation).

When lamps 1, 2 are started and transistor switch Q, is in the openposition, the path of current flow is from the positive input terminallead 72 through the shunt resistor R through the parallel branchincluding capacitor C and the parallel branch including output lead 74,lamps 1, 2 and the inductor L the diode D the inductor L and to theinput terminal lead 71. During this switching mode, the operation oflamps 1 and 2 is controlled by the combined excursions of energy fromthe power source and from the inductor L Further, these excursions ofenergy also charge the capacitor C Since the energy released from theinductor L is in the form of a decaying current, the voltage drop acrossthe shunt resistor R decreases and the tunnel diode TD is again reset toits low voltage state when its voltage reaches the valley point value.Accordingly, transistor Q, is again switched off and the transistor Q;is turned on to start another switching cycle. The repeated excursionsof energy are controlled by the switching action of the transistorswitch Q which is driven by a feedback signal from the line currentsupplied at the positive input lead 72.

During the switching mode when transistor switch Q; is closed or in itslow impedance state, the lamps 1 and 2 are energized by an excursion ofenergy from the capacitor C Energy stored in the capacitor C in thepreceding switching mode is now discharged, and the path of current flowis in a closed loop which includes capacitor 0;, output lead 74, lamps 1and 2, output lead 73, and the inductor L The inductor L prevents theenergy stored in the capacitor C; from being suddenly discharged throughthe lamps. The diode D prevents capacitor C from discharging throughtransistor Q; When transistor Q is closed.

In a voltage step-up arrangement, it will be understood that the normaloperating voltage of the lamps 1 and 2 must be above the DC. supplyvoltage. Unless the normal operating voltage is greater than the DC.supply voltage, the output current will continue to rise until a failureoccurs.

III. APPARATUS FOR OPERATING ELECTRIC DIS- CHARGE LAMPS WITHBIDIRECTIONAL CUR- RENT EMPLOYING THE IMPROVED CURRENT LEVEL SWITCHINGARRANGEMENT OF THE INVENTION A. Description of apparatus shown inFIGURES 16 and 17 In the preceding Sections I and II, I have describedembodiments of my invention wherein electric discharge lamps wereoperated with a chopped unidirectional or unipolar current. By way offurther exemplification of my invention, I shall now describeembodiments of my invention for operating electric discharge lamps witha bidirectional current.

Referring now to FIGURE 16, I have shown therein a schematic circuitdiagram of an apparatus embodying the current level switchingarrangement of the invention for operating an electric discharge lamp11. When a bidirectional current is applied across input leads 81, 82, abidirectional current is applied across output leads 83, 84 and lamp 11.The current level switching function in accordance with the invention iscarried out by a bidirectional switch which is connected in circuit withinput terminal lead 81 and with an inductor L A second bidirectionalswitch 86 is connected in series with bidirectional switch 85 and acrossinput terminal leads 81, 82 by leads 87, 88.

The bidirectional switches 85, 86 are synchronously opened and closed bya feedback control 89 whch is coupled with bidirectional switches and 86and by means of electrical leads 90, 91 and 92, 93, respectively. Itwill be seen that the feedback control 89 is also connected across ashunt resistor R by electrical leads 94, 95.

Essentially, the bidirectional switches 85 and 86 function as a singlepole double throw switch. When one of the switches is driven to the openposition, the other is synchronously driven to a closed position. Thus,when switch 85 is driven to the open position, the power supply isdisconnected from the lamp circuit. During the interval that the powersupply is disconnected, switch 86 is in a closed position, a path isprovided for the discharge of energy stored in the inductor L and theconduction of the lamp 11 is maintained during this interval by thisenergy. This path includes the inductor L output lead 83, lamp 11,output lead 84, the shunt resistor R the electrical lead 88, thebidirectional switch 86 and electrical lead 87.

When switch 85 is driven to a closed position, switch 86 issynchronously driven to an open position by the feedback control 89, andthe power source is now connected in circuit with lamp 11. During thisswitching mode, energy is supplied to lamp 11 and stored in the storageelement or inductor L A path for the current supplied from the powersource is provided by input lead 81, bidirectional switch 85, inductor Loutput lead 83, lamp 11, output lead 84, shunt resistor R and input lead82. Thus, in each half cycle of the alternating current supply, thecurrent level switching action of the bidirectional switches 85 and 86limits the current supplied to lamp 11 and provides the ballastingaction required because of the negative resistance characteristic of theelectric discharge lamp. Further, the current level switching actionprovides a stepdown in the supply voltage. In other words, the dischargelamp 11 is operated at a voltage that is less than the average voltageapplied across the input terminal leads 81, 82.

It will be appreciated that the apparatus of the invention can bereadily adapted to provide either a step-down or a step-up in the supplyvoltage. In FIGURES l8 and 19, I have illustrated an embodiment of theinvention wherein the alternating voltage of the power supply is steppedup to operate the electric discharge lamp with bidirectional current.

Having reference now to the schematic diagram of FIGURE 17, I have showntherein detailed circuit diagrams of the bidirectional switches 85, 86and the feedback control 89 corresponding to the blocks 85, 86 and 89shown in FIGURE 16. Referring more specifically to the feedback control89 as shown in FIGURE 17, it will be seen that a feedback signal issupplied to the feedback control 89 through leads 94, 95 to resistor Rand steering diodes D and D Steering diode D is forward biased duringthe positive half cycle while steering diode D is forward biased duringthe negative half cycle.

The opening and the closing of the bidirectional switches 85, 86 aresynchronized in the positive half cycle by the pulses generated acrosssecondary windings S and S of pulse transformers T and T A negativepulse across secondary winding S and the input leads 90, 91 ofbidirectional switch 85 will cause the normally closed transistor Q tobe driven to an open position and a negative pulse across the secondaryWinding S and input leads 92, 93 will cause the normally open transistorswitch Q to be driven to a closed position.

It will be noted that primary winding P of the pulse transformer T andthe primary winding P of pulse transformer T are connected in parallelwith each other in a loop which includes a resistor R and tunnel diodeTD Similarly, the primary windings P and P of the pulse transformers Tand T are connected in parallel with each other and in a loop whichincludes a resistor R and tunnel diode TD It will be observed thattunnel diode TD is poled so that when it is switched to a high voltagestate during the negative half cycle, the change in current through theprimary windings P and P causes a negative pulse to be induced acrossthe secondary windings S and S Having more particular reference now tothe bidirectional switches 85, 86 as shown in FIGURE 17, these circuitswill now be more fully described. A tunnel diode TD, and a PNPtransistor Q comprise a bistable flip flop circuit for driving thetransistor switch Q Similarly, tunnel diode TD; and the PNP transistorsQ and Q comprise a bistable flip-flop circuit for driving the transistorswitch Q Terminals 97 and 98 are provided for connection to a suitablebias supply such as a negative 10 volt D.C. supply (not shown). A Zenerdiode D and resistors R and R control the bias conditions for tunneldiode TD Similarly, a Zener diode D and resistors R and R perform asimilar function for the bidirectional switch 85. Resistors R R R limitbase drive current to transistors Q Q and Q respectively. The

Zener diodes D and D regulate the bias voltage since the Zener diodes DD start conducting when the voltage applied at their terminals reachesthe breakdown voltage of the Zener diodes D D and they continue toconduct varying amounts of current while the voltage across the Zenerdiodes D D remains substantially fixed at the breakdown voltage.

In order to provide for DC. isolation and pulse coupling of thesecondary winding S a capacitor C is connected in circuit therewith.Similarly, a capacitor C is connected in circuit with the secondarywinding S Diodes D D D are connected with the base electrodes oftransistors Q Q and Q Diode D prevents the voltage drop across thecollector and emitter electrodes of transistor Q when transistor Qconducts from driving the base-emitter junction of transistor switch Qto a conducting state. Similarly, diode D prevents the voltage dropacross the collector and emitter elec trodes of transistor Q fromdriving transistor switch Q Diode D performs a similar function fortransistor Q The resistors R R and R provide a path for dischargecurrent when the transistors Q Q and Q are switched to the openposition. Zener diodes 30 and 31 connected across the emitter andcollector electrode of the transistor switches Q and Q respectively,protect the transistor against excess transient voltages.

It will be noted that four diodes D D D D are connected in a bridgearrangement across the transistor switch Q to provide for abidirectional switching action. Thus, diodes D and D provide a paththrough transistor Q during the positive half cycle and diodes D and Dprovide a path for current flow through transistor switch Q during thenegative half cycle. Similarly, in the bidirectional switch 86 diodes Dand D are forward biased during the positive half cycle of thealternating current supply while diodes D and D are forward biasedduring the negative alternation. Although in this illustrated embodimentof the invention, two driven switches were employed, it will be apparentthat a single switching means functioning as a single throw double poleswitch may be used.

B. Discussion of the operation 0 the apparatus shown in FIGURES 16 and17 During operation, the normally closed bidirectional switchrepetitively disconnects and connects the power supply from the lampcircuit without regard to polarity of the power supply, the intervalthat bidirectional switch 85 is in the open position and disconnects thepower supply from the lamp circuit, bidirectional switch 86 issynchronously driven to a closed position. During the interval thatswitch 85 is open, current to lamp 11 will flow in a path which includesthe inductor L output lead 83, lamp 11, output lead 84, lead 88, switch86 and lead 87. Lamp 11 receives energy from the inductor L during thisinterval. When bidirectional switch 85 is closed, bidirectional switch86 is synchronously driven to the open position. During this switchingmode, lamp 11 is directly energized from the alternating power supplythrough switch 85, inductor L and resistor R Also, during this interval,energy is stored to the inductor L The path of current flow comprises aclosed loop which essentially includes the power source, input terminallead 81, switch 85, inductor L output lead 83, lamp 11, output lead 84,shunt resistor R and input terminal lead 82.

Referring now more specifically to the schematic circuit diagrams of thebidirectional switches 85, as and the feedback control 89, the manner inwhich the switches 85 and 86 are synchronously driven will be more fullyexplained. Let us consider an arbitrary positive alternation of thealternating current supply. The voltage applied at input terminal lead81 is assumed to be positive during this alternation. As the voltagebegins the excursion from zero towards the positive peak, switch 85 isin its closed position, and switch 86 is in its open position. Tunneldiode TD of the feedback control 89 is in a low

1. AN APPARATUS FOR OPERATING AN ELECTRIC DISCHARGE LAMP FROM A POWERSOURCE, SAID APPARATUS COMPRISING AT LEAST ONE ENERGY STORAGE ELEMENT, ASWITCHING MEANS, DRIVER MEANS RESPONSIVE TO A PREDETERMINED INCREASE INTHE ENERGY SUPPLIED TO THE ELECTRIC DISCHARGE LAMP TO DRIVE SAIDSWITCHING MEANS TO A HIGH IMPEDANCE STATE AND RESPONSIVE TO APREDETERMINED DECREASE IN SAID ENERGY TO DRIVE SAID SWITCHING MEANS TO ALOW IMPEDANCE STATE, AND CIRCUIT MEANS INCLUDING INPUT LEADS FORCONNECTION WITH THE POWER SOURCE AND OUTPUT LEADS FOR CONNECTION WITHTHE LAMP TO SUPPLY THE OUTPUT OF THE APPARATUS TO THE LAMP, SAID CIRCUITMEANS INCLUDING MEANS TO PROVIDE A PATH FOR THE SUPPLY OF ENERGY FROMSAID ENERGY STORAGE ELEMENT TO SAID LAMP WHEN SAID SWITCHING MEANS ISDRIVEN TO ONE STATE AND TO PROVIDE A PATH FOR THE SUPPLY OF ENERGY FROMTHE POWER SOURCE TO THE LAMP AND FOR THE STORAGE OF ENERGY IN SAIDSTORAGE ELEMENT WHEN SAID SWITCHING MEANS IS DRIVEN TO THE OTHER STATEWHEREBY THE SAID SWITCHING ACTION CONTROLS THE OPERATION OF THE LAMP ANDENERGY IS SUPPLIED TO THE LAMP DURING BOTH STATES OF SAID SWITCHINGMEANS.